Recently, many efforts are made to research and develop the OFDM(A) system suitable for fast data transfer in wire/wireless channel. In the OFDM(A) system, a plurality of carrier waves having mutual orthogonality are used to raise a frequency use efficiency. A process of modulating/demodulating a plurality of the carrier waves in a transmitting/receiving end brings about the same result of performing IDF(inverse discrete Fourier transform)/DFT(discrete Fourier transform) to enable fast implementation using IFFT(inverse fast Fourier transform)/FFT(fast Fourier transform).
A principle of PFDM is to reduce relative dispersion in a time domain by a multi-path delay spread in a manner of dividing a fast data stream into a plurality of slow data streams and simultaneously transmitting the slow data streams via a plurality of subcarriers to increase a symbol duration. So, data transmission according to the OFDM system is carried out by a unit of transmission symbol.
Since modulation/demodulation in the OFDM system can be collectively handled for all subcarriers using DFT (discrete Fourier transform), it is unnecessary to design a modulator/demodulator for each individual subcarrier.
FIG. 1 is a conceptional diagram of an OFDM modulator/demodulator.
Referring to FIG. 1, a data stream inputted in series is converted to parallel data streams amounting to the number of subcarriers and then transforms each of the parallel data streams by Inverse Discrete Fourier Transform. And, IFFT (inverse fast Fourier transform) is used for fast data handling. The inverse-discrete-Fourier-transformed data is converted to serial data gain and the serial data is then transmitted though frequency conversion. And, a receiving side receives the corresponding signal and then demodulates the received signal in a reverse process.
The related art OFDM wireless communication system tunes time synchronization and frequency synchronization of a base station to those of a user equipment by means of a preamble signal periodically inserted in a data frame.
FIG. 2 is a diagram of a data frame in an OFDMA wireless communication system according to a related art, in which a horizontal axis corresponds to a time axis of symbol unit and in which a vertical axis corresponds to a frequency axis of subchannel unit. And, the subchannel means a bundle of a plurality of subcarriers.
Referring to FIG. 2, all subcarriers are allocated to a preamble to be modulated. And, the preamble is inserted with a uniform interval to become a reference point of entire time. Since all signals transmitted/received between a base station and a user equipment are inserted by taking the preamble as a reference, acquisition of the preamble is the most basic and important work for mutual data communications. For instance, if a moment that a user equipment attempts to tune its time synchronization to that of a base station corresponds to a (k+13)th symbol in FIG. 1, the user equipment is unable to tune its time synchronization to that of the base station until the (k+30)th symbol. The user equipment acquires a preamble at the (k+30)th symbol and then tunes its time synchronization. Since one attempt is not enough to tune the time synchronization satisfactorily in general, the user equipment waits for a next preamble to tune the time synchronization more precisely and repeats this process until a prescribed condition is met.
However, since the preamble is inserted with the uniform interval, it takes a long time for the user equipment to tune the time synchronization until the preamble is read. Since it is difficult to tune the time synchronization with one acquisition of the preamble, the corresponding process needs to be repeated several times. Hence, it takes to much time to acquire the preamble. Moreover, if it takes a much time to tune the time synchronization, a preparation for communications is delayed, the power consumption of the user equipment is wasted, and a connection delay to a neighbor target base station in handoff is prolonged to degrade a quality of communications.